Kathleen Genevieve Ledecky (/ləˈdɛki/ ^ⓘ lə-DEK-ee; born March 17, 1997) is an American competitive swimmer. She is the most decorated female swimmer in history and the most decorated American woman in Olympic history, with a total of 14 Olympic medals, including nine golds. She shares the record for the most Olympic gold medals won by a woman with Larisa Latynina and ranks as the fifth-most decorated Olympian of all time. She is regarded as the greatest female swimmer of all time.

Ledecky has won a record 18 individual gold medals at the World Aquatics Championships, and is the world record holder in the women's 800- and 1500-meter freestyle, both long course and short course, as well as the former world record holder in the women's 400-meter freestyle (long course). In her international debut at the 2012 London Olympic Games as a 15-year-old, Ledecky unexpectedly won the gold medal in the women's 800-metre freestyle. Four years later, she left Rio de Janeiro as the most decorated female athlete of the 2016 Olympic Games, with four gold medals, one silver medal, and two world records.

At the 2020 Olympic Games, Ledecky emerged as the most decorated U.S. female athlete and became the first American female swimmer to win an individual event in three straight Olympiads. In 2025, she won gold in the 800 meter World Championship, becoming the first swimmer—male or female—to win seven World Championship gold medals in the same event. In total, she has won 54 medals (40 golds, 11 silvers, and 3 bronzes) in major international competitions, spanning the Summer Olympics, World Championships, and Pan Pacific Championships. During her career, she has broken 17 world records.

Ledecky's success has earned her Swimming World's Female World Swimmer of the Year a record five times. Ledecky was also named Associated Press Female Athlete of the Year in 2017 and 2022, international female Champion of Champions by L'Équipe in 2014 and 2017, United States Olympic Committee Female Athlete of the Year in 2013, 2016 and 2017, Sportswoman of the Year by the Women's Sports Foundation in 2017, and the ESPY Best Female Athlete in 2022. In 2024, she was awarded the Presidential Medal of Freedom by President Joe Biden.

Ledecky was born in Washington, D.C. and raised in the suburb of Bethesda, Maryland, the daughter of Mary Gen (née Hagan) and David Ledecky. Her Czech-born paternal grandfather Jaromír Ledecky arrived in New York City on September 8, 1947, as a student. He later became an economist and married an Ashkenazi Jewish woman named Berta Ruth Greenwald in Brooklyn, New York, on December 30, 1956. Through her paternal grandmother, Ledecky has relatives who were murdered in the Holocaust. Ledecky's mother is of Irish descent. She was raised Catholic and continues to practice the faith, often praying the Hail Mary before her races. Her uncle Jon Ledecky is a businessman and a co-owner of the NHL team New York Islanders.

Ledecky began swimming at the age of six under the influence of her older brother, Michael, and her mother, who swam for the University of New Mexico. In Bethesda, she attended Little Flower School through eighth grade and graduated from Stone Ridge School of the Sacred Heart in 2015. During her high-school swimming career, Ledecky twice set the American and US Open record in the 500-yard freestyle, and she twice set the national high-school record in the 200-yard freestyle.

During the summers prior to 2012, she swam for Palisades Swim Team in Cabin John, Maryland. During the summer of 2012, she trained with the Nation's Capital Swim Club (formerly the Curl Burke Swim Club) under coach Yuri Suguiyama. Following Suguiyama's departure to coach for the University of California, Berkeley, she continued to train with the Nation's Capital Swim Club under coach Bruce Gemmell through the 2016 Summer Olympics. Ledecky accepted an athletic scholarship to Stanford University, where she swam for coach Greg Meehan's Stanford Cardinal women's swimming team.

In December 2016, Ledecky was chosen as one of the sponsors of the US Navy aircraft carrier USS Enterprise alongside Olympic gold medal-winning gymnast Simone Biles. They were the first Olympians to be given this honor.